In an image forming apparatus employing an electrophotographic system such as a copying machine, a printer, or a multi-function peripheral (MFP), a toner image formed on a photoconductive drum is transferred onto a sheet to perform printing. The transfer is performed in a transfer region near the outer circumference of the photoconductive drum. The sheet before the printing is stored in a paper feeding tray or the like. During the printing, the paper is picked up from the paper feeding tray and led to the transfer region through a conveying path configured by several rollers.
While the sheet is conveyed from the paper feeding tray to the transfer region, a paper jam may occur.
In the past, when a paper jam occurs, the paper jam is detected by using plural paper detection sensors arranged on a sheet conveying path. The respective paper detection sensors detect passing timing of the leading end or the trailing end of a sheet conveyed along the sheet conveying path. When the passing timing exceeds an allowable limit value set in advance, the paper detection sensors determine that a paper jam occurs.
However, a cause of a paper jam that occurs during sheet conveyance may be present in a place other than a place where the occurrence of the paper jam is actually detected. For example, because a sheet fed by a paper feeding unit is fed in a skewed state, the sheet may be wrinkled during conveyance or, in the worst case, a paper jam may occur during the conveyance. In such a case, a real cause of the occurrence of the paper jam is not present in a place where the paper jam occurs but is present in a place where the skew of the sheet occurs. However, the image forming apparatus in the past does not have means for notifying a user of the occurrence of the paper jam and the occurrence of the skew in association with each other.
JP-A-2006-16089 discloses a technique for determining detection timing obtained from plural sheet detection sensors set in different positions in a conveying direction, storing the detection timing as a conveyance history when the detection timing exceeds a predetermined value set in advance, and narrowing down, using the conveyance history, places as causes of occurrence of a paper jam including positions other than a position where the paper jam actually occurs. However, JP-A-2006-16089 does not disclose a technique for detecting occurrence of sheet skew.
On the other hand, for example, JP-A-07-276725 discloses a technique for detecting occurrence itself of sheet skew. However, JP-A-07-276725 does not disclose a technique for associating the occurrence of the sheet skew with occurrence of a paper jam. Further, in a skew detecting method disclosed by JP-A-07-276725, it is necessary to add a separate sensor for skew detection. This leads to an increase in cost of an apparatus.